Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fat-like substances, referred to as atheroma or plaque, on the walls of blood vessels. Such deposits occur both in peripheral blood vessels that feed limbs of the body and coronary blood vessels that feed the heart. Localized accumulation of deposits within regions of the blood vessels may result in stenosis, or narrowing of the vascular channel. When this occurs, blood flow is restricted and the person's health is at serious risk.
Numerous approaches for treating such vascular deposits have been proposed, including balloon angioplasty, in which a balloon-tipped catheter is used to dilate a stenosed region within the blood vessel; atherectomy, in which a blade or other cutting element is used to sever and remove the stenotic material; laser angioplasty, in which laser energy is used to ablate at least a portion of the stenotic material; and the like.
Where removal of vascular deposits is not desired, endovascular grafts have been developed to enlarge an occluded vessel lumen, and to hold tissue in place or to provide a support for a graft while healing is taking place. Intraluminal endovascular grafting has been shown to provide an alternative to conventional vascular surgery. Endovascular grafting involves percutaneous insertion of a prosthetic graft into a blood vessel, and such devices are generally delivered via a catheter to a region of interest within the vascular system. These techniques have the clear advantage over conventional surgery of eliminating a need for surgically exposing, replacing, incising, removing, or bypassing a defective blood vessel. Several structures have been shown to have use as intraluminal prosthetic grafts. These structures include coiled stainless steel springs, helically wound coil springs, and expanding stainless steel stents.
In order to apply certain intervention techniques more effectively, a variety of vascular imaging devices and methods may be employed. Of particular interest to the present invention, imaging catheters having ultrasonic transducers at their distal ends have been employed to produce images of the stenotic region from within the blood vessel.
A number of specific designs for ultrasonic imaging catheters have been described. An early design is illustrated in U.S. Pat. No. 4,794,931, where the mechanical components of the imaging system are located within a housing at the distal end of the catheter. The housing includes a fixed guidewire at its distal tip, which is used to position the catheter within the vascular system. While the use of such fixed-guidewire designs can provide excellent image quality, under some circumstances it is desirable to use an "over-the-wire" design where the catheter may be introduced over a separate (movable) guidewire. The use of a movable guidewire offers certain advantages, including improved steering capability through narrow regions and easier catheter exchange, e.g., substitution of an interventional catheter for the imaging catheter.
Various designs for removable and non-removable imaging cores have also been described. Specific examples are set forth in Sieben et al., U.S. Pat. No. 5,243,988, and in Sieben et al., U.S. Pat. No. 5,353,798, both of which are incorporated herein by reference. These devices generally include an imaging core designed to operate within a guidewire lumen of therapeutic catheters which accept a guidewire of 0.018 inch or smaller. In one embodiment, the device consists of a fixed, non-removable proximal hub apparatus, which is not disconnectable from the imaging core. The main body of the device consists of the drive shaft, and this shaft provides for ease of loading the imaging core into the therapeutic catheter, and steady rotational movement in order to obtain an image without distortion.
Exchanging the imaging catheter for an interventional or other catheter within a patient's vascular system is time consuming and may be injurious to the patient. It is desirable therefore to combine imaging and interventional capabilities in a single catheter system. A design for an ultrasonic imaging catheter having a balloon angioplasty device is described in U.S. Pat. No. 5,117,831. One depicted embodiment uses a fixed guidewire and is thus subject to the disadvantages noted above. Another embodiment has a guidewire movable through the ultrasonic imaging transducer and is associated drive shaft. This requires that the transducer and is associated drive shaft and not movable along the length of the catheter.
To be able to cross very narrow lesions, the diameter of the catheter should be as small as possible at its distal end. Furthermore, the need to move the catheter body within the patient should be minimized. The blood vessel interior is delicate, may be weakened by disease, and is therefore susceptible to injury from movement of the catheter body within it.
Despite the availability of devices for imaging a body passageway as described above, there is a dearth of techniques for guided placement of prosthetic devices, including stents, within a body passageway, including the vascular system. The vasculature of the human body is highly curved and includes many branches in vessels. It is moreover undesirable to place a prosthetic device in a branched segment of a blood vessel for at least two reasons. First, a prosthetic device such as a stent will, in time, accumulate endothelial cell growth which will clog the wire mesh of the stent. If the stent is located at a vessel branch point, then the stent, once clogged, will act as an impermeable barrier to blood flow into the vessel which branches away from the main vessel. Second, if a later episode of vascular disease occurs in the vessel which branches away from the main vessel, then the stent which is located at the vessel branch point will block access to the branched vessel, and will thereby preclude the later use of a vascular catheter as an interventional therapy for this branched blood vessel.
The present techniques for guided placement of a prosthetic device within a vessel rely on X-ray imaging or fluoroscopy to visualize the location of the stent, and angiography using a radiopaque substance to visualize the branch points of blood vessels. This technique suffers from the serious disadvantage that the prosthetic device or stent is detectable by fluoroscopy only when the device is constructed from metal which is sufficiently thick. However, the thickness of metal needed for fluoroscopic detection causes the prosthetic device to be too stiff to safely navigate the tortuous passageways of blood vessels. For this reason, the presently available techniques for guided placement of stents cannot be utilized with the most widely used and most preferred stents; flexible stents adapted for entry into narrow and highly curved blood vessels, such as the Palmaz-Schatz stent available from Johnson & Johnson. For these reasons, approximately 95% of the stents which are in wide use today are not radiopaque.
For the reasons stated above, it would be advantageous to provide a catheter system having a balloon angioplasty system with a prosthetic device such as a vascular graft or stent associated therewith, in combination with visualization capabilities for guided placement of the prosthetic device within a region of interest within the vascular system. Such delivery should be accomplished with a minimum repositioning of the catheter body within the blood vessel. Additionally, the catheter should be as narrow as possible at its distal end to allow for entry into and through narrow and tortuous regions of the patient's vascular system.